I have a few experiences with volunteering, not all good. I'll start by saying that when I was in college, my sorority had an amazing dedication to service - we were volunteering for something every month. It was a nice way to get involved in our community and we really had a lot of fun.
Let's see .... the rundown .... I've volunteered with Habitat for Humanity a few times and they are probably my favorite organization. It's a very visceral moment when you get to meet the family for whom you are building - very sweet and powerful.
At another point, when I was waiting for something professional to open up I decided to work non-professionally and then volunteer as an archivist for a local history organization. This was truly fascinating. It was a community I knew nothing about so accessioning the photographs of their local history was like a puzzle you could never figure out. Riveting. And yet, not really my bag. I love digging through old stuff, but I didn't really care to do it for a living .... unless it was as one of those home organizers. That would be awesome.
I do agree that if we volunteer too much, it will hurt our chances to be professional. There was one place I lived that was very hard to get a job as the economy was haggard and the jobs scarce. I walked over to my neighborhood library branch and asked about their hiring -- they said that they didn't hire anyone who hasn't volunteered for at least two years. Yes, they said that. I said, well, that would be awesome except I actually need to eat food and pay that student loan I have over the next two years, so thank you anyway, but no thanks. WTH? Really? That's arrogance at it's worst and it made me mad enough to move to a place where the industry was hiring.
Other than these kinds of volunteering, something I love to do is charity walks and cycling events. They are fun, you raise money for charity, workout a bit, and have a really good time chatting with people you would never normally meet. I think I'm a little too selfish with my time at this point to do much else.